This invention relates to an intake air amount control system for internal combustion engines, and more particularly to a system of this kind which is adapted to learn values of the control amount used for controlling the intake air amount during feedback control of idling rotational speed of the engine.
A control system for controlling the amount of intake air supplied to an internal combustion engine is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 61-258947, which comprises valve means for regulating the amount of intake air supplied to the engine through the intake pipe, control means for determining the difference between a desired idling rotational speed of the engine and the actual rotational speed of same and controlling the valve means in a feedback manner responsive to the determined difference such that the actual engine rotational speed becomes equal to the desired idling rotational speed, and learning means for calculating an average value of values of a feedback control amount applied during the feedback control and adapting the calculated average valve as a reference value of the control amount, i.e. learn the reference value, and wherein the reference value is applied as an initial value of the control amount when the next feedback control is started.
The valve means generally comprises a control valve which is arranged in a bypass passage bypassing a throttle valve in the intake pipe for supplying auxiliary air to a downstream side of the intake pipe. The control valve usually comprises a linear solenoid valve which opens to a degree proportional to the amount of driving current applied thereto.
The learning of the reference value of the control amount for feedback control should be carried out when the control system including the engine is in a steady state. If the learning is carried out when the control system is in an unsteady state such as a transient state, or in a state where the control amount assumes values different from those assumed during the feedback control, such as when an external load is applied to the engine, e.g. by the air conditioner installed in the vehicle equipped with the engine, the learned reference value will assume an improper value. Particularly, when the vehicle is running at a high altitude, there is a decrease in the difference .DELTA.P between atmospheric pressure P.sub.A and absolute pressure P.sub.BA within the engine intake pipe on the downstream side of the throttle valve, so that the speed of intake air flow lowers. Consequently, the actual flow rate of intake air is largely different with reference to the amount of driving current (command current) for opening the control valve from a normal value. Therefore, if learning of the reference value of control amount is carried out with the decreased pressure difference .DELTA.P, the learned reference value will assume an improper value.
More specifically, referring to FIG. 4, let it be assumed that an amount Q1 of air is to be supplied to the engine. When the pressure difference .DELTA.P assumes a normal value of 300 mmHg or larger, the required command current amount I.sub.CMD for the control valve is I.sub.1. However, when the pressure difference .DELTA.P is 200 mmHg, the required command current amount I.sub.CMD increases to I.sub.2. When the pressure difference .DELTA.P is 100 mmHg, the required command current amount I.sub.CMD increases to a value as large as I.sub.3. With such a characteristic of command current amount vs. air amount, when the pressure difference .DELTA.P is as small as 100 mmHg, the command current amount I.sub.3 is learned as a reference value of the feedback control amount. If this learned reference value is applied at the start of the next feedback control executed when the pressure difference .DELTA.P assumes a normal value of 300 mmHg or larger, an excessive air amount Q3 will be supplied to the engine based on the learned reference value.
The known control system carries out learning of the reference value even when the pressure difference .DELTA.P is small, so that an incorrect or excessive amount of air is supplied to the engine during the next feedback control executed when the pressure difference .DELTA.P is normal or large.